With a few exceptions, tradition and technology have favored transillumination pulse oximetry in the operating theater. The principle of operation of the pulse oximeter is fairly simple but is arguably the most important development in anesthesia monitoring in the twentieth century. Two wavelengths of light (usually 660 nm and 940 nm) are used to spectrophotometrically determine the ratio of oxidized to reduced hemoglobin noninvasively as well as to determine the pulsatility of blood plethysmographically. Presently, the most common application of this in the operating theater is via transillumination through the capillary bed of a peripheral digit. However, it is not unusual for multitrauma and thermally injured patients to either have severe peripheral vasoconstriction or to have severely damaged (or missing due to amputation) peripheral vascular beds. Reflectance oximetry rather than transillumination oximetry was the earliest investigative form of the technique. Transillumination pulse oximetry, without question, is the most effective form when oximetry is obtained through skin. However, when skin is not interposed as a barrier to capillary bed access, reflectance pulse oximetry easily can be achieved with very accurate results. The effect is achieved by the backscattering of incident bispectral light that traverses and, on reflection from nonabsorptive collagenous tissues, retraverses formed elements in the blood back to the oximetric detector. Rather than superseding transillumination pulse oximetry, this technique broadens the scope of possible monitoring sites, adding to the clinician's armamentarium.
Conventional pulse oximetry in the severely burned patient can be a significant hallenge, yet this monitoring data is vital in operating room and intensive care settings. Most current oximetric approaches depend upon available peripheral sites permitting transillumination oximetry and indeed, this method is sufficient for most surgical conditions and procedures. Unfortunately, patients with severe burns often have few sites for the effective placement of the transilluminating pulse oximeter sensor. In addition, these patients often have severe circulatory compromise rendering the peripheral pulse oximeter less efficient. A variety of studies have shown that central pulse oximeters are more reliable and rapidly responsive than peripheral pulse oximeters.
Reflectance oximetry can be a useful tool where a capillary bed is easily accessible. Indeed, it is used commonly and effectively among intrapartum and neonatal patients whose capillary beds are easily accessed through their skin. The technique has also been applied to adult and pediatric burn patients by placing the reflectance sensor in wounds or over hyperemic sites such as healed partial thickness burns.
Prior pulse oximeter sensors inserted through the mouth are usable only when the patient is under general anesthesia. These pulse oximeter sensors are inserted to reach the larynx area, for example, U.S. Pat. No. 5,282,464 to Brain et al. Another known method uses transillumination pulse oximetry of the posterior tongue, but this method may not be used with a patient, who is awake, for example, U.S. Pat. No. 5,205,281 to Buchanan. Also, the posterior tongue is not the most accessible body part to take oximetric measurements.
Recent studies indicate that oral pulse oximetry is a superior modality when compared to peripheral transillumination pulse oximetry. A variety of studies have shown that oral pulse oximeters are more reliably and rapidly responsive than peripheral pulse oximeters. However, these studies use oral transillumination pulse oximetry, held in place via complex devices or pieces of improvised malleable metal. Oral secretions, equipment failure, and placement difficulty often render these techniques ineffective.
There are other often overlooked capillary beds readily accessible in most adult burn patients that are as amenable to reflectance oximetry as the forehead of the premature infant. The buccal surface, posterior soft palate, hard palate and proximal posterior pharynx of a burned patient are seldom compromised no matter how severe the burn, and the capillary beds are very close to the surface in those areas. Transillumination pulse oximetry of the tongue and cheek has been documented as a viable method of monitoring, but not everyone has the equipment available to place a transilluminating pulse oximeter on the tongue or cheek. A reflectance pulse oximeter sensor has the bispectral emitter and the detector in a side-by-side configuration rather than in opposition. The device may be placed flat upon a suitable capillary bed and it thus becomes a reflectance pulse oximeter sensor.
The nasal mucosa and the posterior pharynx contain rich capillary beds ideal for reflectance pulse oximetry. Known pulse oximeters are not suitable for use in the nares as they tend to block the nasal passage thus constricting the patient's breathing. In addition, they are prone to difficulties when their electrical components are exposed to liquid, moisture, bodily fluids, and/or surgical fluids. Since they rely on transillumination they also tend to be difficult to hold in place.
Notwithstanding the usefulness of the above-described devices, and the above-identified recognized viability of transilluminating buccal pulse oximetry, a need exists for a more convenient method for obtaining oximeter readings from a subject.